On the anomalous charging and discharging currents in LDPE under high electric fields

Dielectric characterization and conduction modeling of polymers are considered important owing to increasing use of polymeric insulation worldwide in power cables for electric power distribution. Electric conduction in polymers is a complex phenomenon and differs considerably at high electric fields where phenomena like space charge formation and charge injection from the electrodes come into the picture. This paper investigates the charge injection during polarization and subsequent discharging of low density polyethylene (LDPE) films at high electric fields (> 10 kV/mm) and under different temperatures. An attempt was made to study the short term Debye-type exponential behavior and long term power law behavior. Apart from the variation in the power law exponent for absorption current with increasing field, anomalies were noted in both types of relaxation currents at high electric fields and temperatures. Evidence of the polarization current anomaly was correlated with space charge behavior of samples. Interestingly, the space charge evolution under step voltage application was observed to be oscillatory in some cases, wherein, the charge trapping inside the dielectric bulk showed an oscillatory profile. The experimental factors which could contribute to anomalous discharging currents have also been thoroughly investigated and reported, which seem to have not been paid attention until now.